Rewrite isr_stub to make use of preprocessor macros

git-svn-id: https://subversion.ews.illinois.edu/svn/sp14-ece391/_projects/group14/trunk@16300 1cec0ab5-dbbe-4d55-bb2a-e14ca091d7d0
Update isr.c to use defines instead of integer constants

git-svn-id: https://subversion.ews.illinois.edu/svn/sp14-ece391/_projects/group14/trunk@16301 1cec0ab5-dbbe-4d55-bb2a-e14ca091d7d0